RFR: JDK-8297449: Update JInternalFrame Metal Border code [v4]

[Harshitha Onkar]

> Updated Metal Border code for JInternalFrame. 
> 
> - Added instanceof check before casting Graphics to G2D.
> - Replaced roundHalfDown with Region.clipRound()

Harshitha Onkar has updated the pull request incrementally with one additional commit since the last revision:

  review changes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11305/files
  - new: https://git.openjdk.org/jdk/pull/11305/files/a3afba44..b1e3557a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11305&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11305&range=02-03

  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11305.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11305/head:pull/11305

PR: https://git.openjdk.org/jdk/pull/11305